1. Field of the Invention
The invention relates to a fuel injection system of the type having a displacable piston unit for boosting the pressure of the fuel delivered to the combustion chambers of an internal combustion engine.
2. Description of the Prior Art
For better understanding of the specification and the patent claims, some terms will now be defined: The fuel injection system of the invention can be embodied as either stroke-controlled or pressure-controlled. Within the scope of the invention, the term stroke-controlled fuel injection system is understood to mean that the opening and closing of the injection opening are done with the aid of a displaceable valve member, on the basis of the hydraulic communication of the fuel pressures in a nozzle chamber and in a control chamber. A pressure reduction inside the control chamber causes a stroke of the valve member. Alternatively, the deflection of the valve member can be effected by means of a final control element (actuator). In a pressure-controlled fuel injection system according to the invention, the valve member is moved counter to the action of a closing force (spring) by means of the fuel pressure prevailing in the nozzle chamber of an injector, so that the injection opening is uncovered for an injection of the fuel out of the nozzle chamber into the cylinder. The pressure at which fuel emerges from the nozzle chamber into a cylinder of an internal combustion engine is called the injection pressure, while the term system pressure is understood to mean the pressure at which fuel is kept available or kept in reserve inside the fuel injection system. Fuel metering means furnishing a defined fuel quantity for injection. The term leakage is understood to mean a quantity of fuel that occurs in operation of the fuel injection system (for instance, a reference leakage) but that is not used for injection and is returned to the fuel tank. The pressure level of this leakage can have a standing pressure, and the fuel is then depressurized to the fuel level of the fuel tank.
A stroke-controlled injection has been disclosed for instance by German Patent Disclosure DE 196 19 523 A1. The attainable injection pressure is limited here to approximately 1600 to 1800 bar by the pressure reservoir chamber (rail) and the high-pressure pump.
For increasing the injection pressure, a pressure booster unit is possible, of the kind known for instance from U.S. Pat. No. 5,143,291 or U.S. Pat. No. 5,522,545. The disadvantage of these pressure-boosted systems is the lack of flexibility of injection and poor quantity tolerance when metering small fuel quantities.
A pressure booster unit disposed in the injector is known from European Patent Disclosure EP 0 691 471 A1. A bypass line for a pressure injection and a pressure chamber of the pressure booster unit are connected in series, so that the bypass line is passable only as long as a displaceable piston unit of the pressure booster unit is not in motion and is fully retracted.
For increasing the injection pressure and the flexibility of the injection, in a common rail injection system, a pressure booster unit is advantageous. To keep the engineering expense and thus the production costs low, controlling the pressure booster unit is done with a simple 2/2-way valve.
A fuel injection system according to the invention is operable to reduce the control quantity during the triggering of the pressure booster unit and for performing a rapid restoration of the piston unit of the pressure booster unit.
By means of the filling valve, an additional filling path is opened up for restoration of the piston unit. The control of the filling valve is effected without an actuator, by means of a pressure difference at the pressure booster unit, in order to keep the engineering expense low.
To achieve a defined pressure difference at the valve body of the filling valve, a throttle restriction can be embodied between the valve body and the guide bore. An additional supply line with a throttle that is preferably kept small serves to initiate the restoration of the piston unit. If the filling valve has a spring and corresponding pressure faces, which can be pressure-actuated by fuel, for switching the filling valve, then the valve body of the filling valve can easily be shifted to the closed position of the filling valve.